Types of organic reactions

Nature of science:

Looking for trends and discrepancies—by understanding different types of organic reactions and their mechanisms, it is possible to synthesize new compounds with novel properties which can then be used in several applications. Organic reaction types fall into a number of different categories. (3.1) Collaboration and ethical implications—scientists have collaborated to work on investigating the synthesis of new pathways and have considered the ethical and environmental implications of adopting green chemistry. (4.1, 4.5)

• SN1 represents a nucleophilic unimolecular substitution reaction and S_N2 represents a nucleophilic bimolecular substitution reaction. S_N1 involves a carbocation intermediate. S_N2 involves a concerted reaction with a transition state.
• For tertiary halogenoalkanes the predominant mechanism is SN1 and for primary halogenoalkanes it is S_N2. Both mechanisms occur for secondary halogenoalkanes.
• The rate determining step (slow step) in an S_N1 reaction depends only on the concentration of the halogenoalkane, rate = k[halogenoalkane]. For S_N2, rate = k[halogenoalkane][nucleophile]. S_N2 is stereospecific with an inversion of configuration at the carbon.
• S_N2 reactions are best conducted using aprotic, non-polar solvents and S_N1 reactions are best conducted using protic, polar solvents.

Electrophilic Addition Reactions:

• An electrophile is an electron-deficient species that can accept electron pairs from a nucleophile. Electrophiles are Lewis acids.
• Markovnikov’s rule can be applied to predict the major product in electrophilic addition reactions of unsymmetrical alkenes with hydrogen halides and interhalogens. The formation of the major product can be explained in terms of the relative stability of possible carbocations in the reaction mechanism.

Electrophilic Substitution Reactions:

• Benzene is the simplest aromatic hydrocarbon compound (or arene) and has a delocalized structure of π bonds around its ring. Each carbon to carbon bond has a bond order of 1.5. Benzene is susceptible to attack by electrophiles.

Reduction Reactions:

• Carboxylic acids can be reduced to primary alcohols (via the aldehyde). Ketones can be reduced to secondary alcohols. Typical reducing agents are lithium aluminium hydride (used to reduce carboxylic acids) and sodium borohydride.

• What role does green and sustainable chemistry, in relation to organic chemistry, play in a global context?

Utilization:

• Organic synthesis plays a vital role in drug design and drug uptake in medicine and biochemistry.
• Nutrition, food science and biotechnology also are underpinned by organic chemistry.

Syllabus and cross-curricular links:
Topics 10.1 and 10.2—organic chemistry
Topic 14.1—covalent bonding
Topic 14.2—hybridization
Option A.5 and A.9—polymers
Aims:

• Aim 6: Three-dimensional visualization of organic compounds using molecular models could be covered.

• Explanation of why hydroxide is a better nucleophile than water.
• Deduction of the mechanism of the nucleophilic substitution reactions of halogenoalkanes with aqueous sodium hydroxide in terms of S_N1 and S_N2 mechanisms. Explanation of how the rate depends on the identity of the halogen (ie the leaving group), whether the halogenoalkane is primary, secondary or tertiary and the choice of solvent.
• Outline of the difference between protic and aprotic solvents.

Electrophilic Addition Reactions:

• Deduction of the mechanism of the electrophilic addition reactions of alkenes with halogens/interhalogens and hydrogen halides.

Electrophilic Substitution Reactions:

• Deduction of the mechanism of the nitration (electrophilic substitution) reaction of benzene (using a mixture of concentrated nitric acid and sulfuric acid).

Reduction Reactions:

• Writing reduction reactions of carbonyl containing compounds: aldehydes and ketones to primary and secondary alcohols and carboxylic acids to aldehydes, using suitable reducing agents.
• Conversion of nitrobenzene to phenylamine via a two-stage reaction.

• Aim 6: A range of experiments of organic synthetic reactions exploring various types of reactions and functional group interconversions could be done. Core techniques of organic chemistry could include reflux, distillation, filtration, purification (including chromatographic techniques), separations and extractions.
• Aim 6: Synthesis (or reaction) in the laboratory of an example of a widely used drug or medicine (eg aspirin) or a household product (eg fading of tomato ketchup—electrophilic addition reaction of bromine).

• Reference should be made to heterolytic fission for S_N1 reactions.
• The difference between homolytic and heterolytic fission should be understood.
• The difference between curly arrows and fish-hooks in reaction mechanisms should be emphasized.
• Use of partial charges (δ+ and δ-) and wedge-dash three-dimensional representations (using tapered bonds as shown below) should be encouraged where appropriate in explaining reaction mechanisms.

• Typical conditions and reagents of all reactions should be known (eg catalysts, reducing agents, reflux etc.). However, more precise details such as specific temperatures need not be included.